1. Field of the Invention
The present invention relates to a non-aqueous electrolyte battery having electrodes formed by integrating active materials by using a binder, and more particularly to an improvement in a binder.
2. Description of Prior Art
In recent years, the performance of electronic equipment has been improved and size reduction and portable structure have been realized as the electronic technology progresses. The explosion in the use of notebook type personal computers and portable telephones raises a demand for a non-aqueous electrolyte battery capable of reducing the size and the weight, enlarging the capacity and realizing a high energy density and satisfactory reliability.
In particular, a non-aqueous electrolyte secondary battery for use in a portable telephone is used both outdoors and indoors. Therefore, the non-aqueous electrolyte secondary battery is used in a very wide temperature range and a rapid change in the temperature of the non-aqueous electrolyte secondary battery takes place. Thus, excellent reliability has been required for the non-aqueous electrolyte secondary battery against temperatures and thermal shock.
In addition to the reliability against temperatures, the non-aqueous electrolyte secondary battery must have significantly satisfactory reliability without deterioration in the charge and discharge cycles performed for a long time.
At present, non-aqueous electrolyte batteries include non-aqueous electrolyte secondary batteries having a cathode made of a composite oxide containing lithium and an anode made of a material which is capable of occluding and discharging lithium ions, and non-aqueous electrolyte primary batteries having a cathode made of a chalcogen compound, from which lithium can be desorbed and into which the same can be inserted, and an anode made of lithium metal.
The electrode of the non-aqueous electrolyte secondary battery is manufactured in such a manner that an active material, that is, a monolithic carbon material, such as graphite or carbon black, is employed as a conductive material and a binder is used to integrate the conductive material. The binder is made of fluororesin, such as polyvinylidene fluoride (hereinafter called "PVdF") or polytetrafluoroethylene (hereinafter called "PTFE"), or an organic polymer, such as fluorine rubber, carboxylmethylcellulose (CMC), methylcellulose (MC) or polyimide.
Among the above-mentioned materials, PVdF and PTFE are employed as the binder for use in the electrodes of the non-aqueous electrolyte battery because of excellent oxidation resistance.
When a fluorine polymer, such as PVdF or PTFE, is employed as the binder, the binding force between the depolarizing mix for a cell and the collector however is too weak. In this case, use of the non-aqueous electrolyte battery in a state where rapid change in the temperature is repeated results in the depolarizing mix being desorbed from the collector. As a result, the capacity of the non-aqueous electrolyte battery is reduced undesirably.
On the other hand, polyimide is a binder exhibiting excellent binding properties. When polyimide is employed as the binder, the force for binding the depolarizing mix and the collector with each other can be enlarged. Thus, even if the non-aqueous electrolyte battery is used in a state where the rapid change in the temperature is repeated, the depolarizing mix is not desorbed from the collector. Therefore, there is a possibility that a non-aqueous electrolyte battery free from reduction in the capacity thereof and thus having excellent reliability can be realized.
Although polyimide exhibits excellent binding properties between the depolarizing mix and the collector, rigid molecular chains of the polymer cause the manufactured electrode to have a rigid characteristic. Therefore, when the foregoing polyimide is used as the binder, the manufactured electrode has a low tolerance to bending. Thus, cracks or the like of the electrode cannot be prevented and therefore the characteristics of the non-aqueous electrolyte battery deteriorate excessively.
In general, polyimide is not dissolved in any organic solvent. Therefore, the polyimide must be made to be soluble in an organic solvent when the polyimide is employed as the binder. Thus, the depolarizing mix is prepared by using thermoplastic polyamic acid or the like soluble in an organic solvent, and then applied to the collector and heated in a drying process so that polyamic acid is converted into imide. Since the polyamic acid is soluble in an organic solvent, excellent productivity can be realized when electrodes of the non-aqueous electrolyte batteries are manufactured.
When polyamic acid is heated so as to be formed into imide, water however is generated. Thus-generated water adversely affects the active material, as a result of which the discharge capacity of the non-aqueous electrolyte battery and the cycling characteristic of the non-aqueous electrolyte deteriorate excessively.